Interactive effects of mulching practice and nitrogen rate on grain yield, water productivity, fertilizer use efficiency and greenhouse gas emissions of rainfed summer maize in northwest China

Highlights

• Mulching practice and N rate effects on maize yield and GHG emissions were explored.

• No significant differences existed in GY, WP, NUE and SNR between RF and SM.

• Straw mulching decreased GWP by 23.1% and GHGI by 29.0% compared with NM.

• GWP and GHGI under RF were 62.1% and 22.4% higher than those under NM.

• Straw mulching with 200 kg N ha⁻¹ was recommended for sustainable maize production.

Abstract

Soil mulching and nitrogen application have long been applied in rainfed agriculture, but their interactive effects on crop productivity and especially environments remain poorly understood. Field experiments were carried out in 2018 and 2019 to investigate the responses of grain yield, water productivity, nitrogen use efficiency, economic benefit and greenhouse gas (N₂O, CO₂ and CH₄) emissions of rainfed summer maize in northwest China to various mulching practices (NM: no mulching, SM: straw mulching, and RF: ridge-furrow cultivation with film mulching on the ridge), nitrogen rates (N1: 100 kg N ha⁻¹, N2: 200 kg N ha⁻¹ and N3: 300 kg N ha⁻¹) and their interactions. The results showed that there were no significant differences in grain yield, water productivity and NUE between RF and SM irrespective of nitrogen rates, but RF and SM significantly increased grain yield by 12.4% and 7.9%, water productivity by 13.7% and 10.2%, and partial factor productivity by 12.1% and 7.2% compared with NM (7620.8 kg ha⁻¹, 18.86 kg ha⁻¹ mm⁻¹ and 45.3 kg kg⁻¹), respectively. SM obtained comparable net income to RF under N2 (1567.0 and 1632.4 USD ha⁻¹) and N3 (1593.9 and 1709.9 USD ha⁻¹). Grain yield and net income increased by 12.0% and 14.8% from N1 (7408.6 kg ha⁻¹ and 1356.6 USD ha⁻¹) to N2, and by only 5.5% and 3.6% from N2 (8297.3 kg ha⁻¹ and 1558.0 USD ha⁻¹) to N3, respectively. Water productivity was increased by 14.3% from N1 (18.6 kg ha⁻¹ mm⁻¹) to N2, but it decreased by 3.3% in 2018 and increased by 3.0% in 2019 from N2 (23.2 and 19.4 kg ha⁻¹ mm⁻¹) to N3. Both SM and RF increased CO₂ emission compared to NM, while N₂O emission decreased under SM and increased under RF. RF increased soil CH₄ absorption, while CH₄ patterns were not consistent under SM. SM significantly decreased global warming potential by 23.1% and greenhouse gas intensity by 29.0% relative to NM (405.4 kg CO₂-eq and 52.0 kg CO₂-eq t⁻¹ yield), but they were significantly increased by 62.1% and 22.4% compared with NM under RF, respectively. Compared to N1 (74.09 kg kg⁻¹), N2 and N3 decreased partial factor productivity by 44.0% and 60.9%, respectively. N rate had little influence on CO₂ and CH₄ emissions, but it significantly promoted N₂O emission. There were significant interacting effects of mulching practice and nitrogen rate on N₂O emission and global warming potential. In conclusion, straw mulching with 200 kg N ha⁻¹ achieved a better balance between agronomic, economic and environmental benefits of rainfed summer maize in northwest China.

Introduction

Rainfed agriculture accounts for more than 70% of the total arable land in north and northwest China, mainly located in the arid and semi-arid regions (Li et al., 2014). Drought and nutrient deficiency are the major factors limiting maize (Zea mays L.) production in these regions (Hu et al., 2019, Xu et al., 2020). Scarce and variable annual precipitation (200–600 mm) is the only water input for rainfed agriculture, about 60% of which occurs between July and September (Chen et al., 2020, Fan et al., 2021, Zheng et al., 2019). In the context of water shortage, straw mulching (SM) and ridge-furrow cultivation with plastic film mulching on the ridge (RF) have been widely adopted to improve water productivity and maize yield by reducing soil evaporation, conserving soil water and regulating soil temperature, organic matter and structure (Li et al., 2013, Chen et al., 2017, Zhang et al., 2017a, Hu et al., 2019, Maneepitak et al., 2019, Wang et al., 2019, Zhang et al., 2021, Zheng et al., 2020, Zheng et al., 2021). Nitrogen (N) amendment is an effective way for promoting maize growth and ultimately enhancing grain yield (Wang et al., 2020). However, the N input for rainfed maize in northwest China averagely amounts to ~300 kg N ha⁻¹, which is much higher than the nitrogen requirement for summer maize (Chang et al., 2014). Although the high N input may guarantee high maize yield, it significantly decreases the nitrogen use efficiency (NUE) and contributes to high soil N residual. The NUE for the wheat-maize cropping system in China was only 16%−18% due to the excessive use of mineral N from 1987 to 2015 (Liang et al., 2018).

Scientists have recently paid more attention to environmental issues worldwide. Ammonia (NH₃) is a major environmental pollutant and a critical precursor for PM_2.5 (Dan et al., 2004, Sun et al., 2018). Its atmospheric deposition to the land surface also contributes to soil acidification, eutrophication and loss of biodiversity (Asman et al., 1998, Hellsten et al., 2008). Agricultural fertilizers are the world’s biggest sources of NH₃, contributing 41.9% to the total emission (Xu et al., 2015, Pan et al., 2016). Global warming due to increasing atmospheric greenhouse gas (GHG) emissions is also a severe environmental issue. Nitrous oxide (N₂O), carbon dioxide (CO₂) and methane (CH₄) are the major GHG and have rapidly increased, especially since the mid-20th century. Agriculture is the second greatest GHG emission source after fossil fuels. Anthropogenic CO₂ emission dominated by agriculture totaled 5575 Mt and increased to 5800 Mt by 2014 (FAO, 2015). Global warming potentials (GWP) of N₂O and CH₄ are 298 and 28 times greater than that of CO₂, and agriculture soil has been estimated to contribute 84% and 52% to the global anthropogenic N₂O and CH₄ emissions, respectively (IPCC, 2014).

The changing soil water and temperature conditions as a result of soil mulching can influence NH₃ and GHG emissions. However, it is not clear yet that how straw and plastic film mulching affect NH₃ volatilization despite several studies on NH₃ volatilization in northwest China or other regions with similar climates (Cai et al., 2002, Zhang et al., 2004, Wang et al., 2011, Zhang et al., 2011, Zhang et al., 2015, Li et al., 2017, Li et al., 2019a). Inconsistent results of straw and plastic mulching on GHG emissions have been observed, even in the same field during various seasons (Chen et al., 2017, Zhou et al., 2017). CO₂ and CH₄ emissions under straw mulching are mostly increase due to the readily available C from straw decomposition, but Chen et al. (2017) found that straw mulching increased the soil’s ability to absorb CH₄. It has been reported that RF increased NO₂ and CO₂ emissions because the increased soil temperature and moisture stimulated nitrifier and/or denitrifier activities (Wrage et al., 2001) and promoted soil mineralization (Li et al., 2004). However, NO₂ and CO₂ emissions have also been decreased largely due to the decreased soil mineral N or the barrier effect of plastic film on the ridge (Gan et al., 2013, Liu et al., 2014). Improving our knowledge of soil mulching effects on maize growth and environmental effects is still highly required for rainfed agriculture.

Nitrogen application can lead to NH₃ air pollution and GHG emissions from mineral N fertilizers. Previous studies showed that applied N was lost by NH₃ volatilization under decreased soil pH conditions due to hydrolysis of granule urea into NH₄⁺, OH⁻ and CO₃²⁻ after N application (Hayashi et al., 2008, Tian et al., 2017). N₂O can be generated from the nitrification and denitrification processes, which are mainly controlled by the soil available substrate (NO₃⁻, NH₄⁺) (Migliorati et al., 2014, Wang et al., 2016, Zhong et al., 2016). Urea is converted into NH₄⁺, OH⁻ and HCO₃⁻ in the presence of water and urease enzymes, and HCO₃⁻ then evolves into CO₂ and water. Therefore, higher NH₃, N₂O and CO₂ are usually emitted from the soil with the increasing nitrogen input (Snyder et al., 2009). However, both increase and decrease of soil oxidation capacity for CH₄ emission have been observed with the increased NH₄⁺ contents from fertilizers (Van den Pol-van Dasselaar et al., 1999, Bodelier and Laanbroek, 2004). An optimal N rate is thus critical not only for sustaining maize yield but also for ensuring good environment.

Soil mulching and nitrogen application have long been used for improving crop productivity in rainfed agriculture, but there remain large uncertainties regarding their comprehensive impacts on environments, such as NH₃ volatilization and greenhouse gas emissions. What’s more important, the interacting effects of mulching practice and nitrogen rate on maize yield and greenhouse gas emissions have not been explored yet. Therefore, the objective of this study was to explore the responses of maize yield, water productivity (WP), N utilization efficiency (NUE), economic benefit, NH₃ ventilation and GHG emissions to various soil mulching practices, N application rates and their interactions. The hypotheses were that: (1) plastic film mulching produces the highest maize yield, WP, NUE and economic benefit but also the greatest GHG emissions; (2) straw mulching obtains comparable maize yield, WP, NUE and economic benefit to plastic film mulching, but it produces low GHG emissions; and (3) the combination of straw mulching and a medium nitrogen rate of 200 N kg ha⁻¹ better balances the agronomic, economic and environmental benefits of rainfed maize.